Hey. it does cover the entire screen, however not completely, it is illuminated (And fully readable.) however has a distinct darker patch in the bottom corner. A quick google image search shows it perfectly.
It is a lot of money and it is not fully efficient. Passable, yes, excellent, no. We have one well-lit corner, a good but not excellent middle area and poorly lit corner. Problem is, for over £50 we should expect excellent light coverage and readability.
Thanks to all who replied. My wife did purchase one in the end and it's so good that I bought myself one as well. although the bottom left corner isn't quite as bright as the rest, it isn't enough to make it hard to read, even in complete darkness. Although they are expensive, it's well worth thee £20 over the cost of the unlighted cover.